The cost that has been charged against income for stock-based compensation is set forth below (in thousands):

	Three Months Ended				Nine Months Ended
	October 2, 2020		September 27, 2019		October 2, 2020		September 27, 2019
Employee stock options	$	2,469	$	2,178	$	7,206	$	5,770
Restricted stock		119		77		273		236
Restricted stock units		415		246		1,238		1,661
Performance stock units		54		—		54		—
Nonemployee stock options		69		57		194		111
Total stock-based compensation expense	$	3,126	$	2,558	$	8,965	$	7,778

The Company recorded stock-based compensation costs in the following categories (in thousands):

	Three Months Ended				Nine Months Ended
	October 2, 2020		September 27, 2019		October 2, 2020		September 27, 2019
Cost of sales	$	30	$	7	$	82	$	43
General and administrative		1,307		1,082		3,601		2,878
Marketing and selling		807		692		2,674		2,553
Research and development		982		777		2,608		2,304
Total stock-based compensation expense, net		3,126		2,558		8,965		7,778
Amounts capitalized as part of inventory		287		258		847		723
Total stock-based compensation expense, gross	$	3,413	$	2,816	$	9,812	$	8,501

The Amended and Restated Omnibus Equity Incentive Plan (“the Plan”) provides for various forms of stock-based incentives. To date, of the available forms of awards under the Plan, the Company has granted only stock options, restricted stock, unrestricted share grants, restricted stock units (“RSUs”) and performance stock units (“PSUs”). Options under the Plan are granted at fair market value on the date of grant, become exercisable generally over a three-year period, or as determined by the Board of Directors, and expire over periods not exceeding 10 years from the date of grant. Certain option and share awards provide for accelerated vesting if there is a change in control and pre-established financial metrics are met (as defined in the Plan). Grants of restricted stock outstanding under the Plan generally vest over periods of one to three years. Grants of RSUs and PSUs outstanding under the Plan generally vest based on service, performance, or a combination of both. On July 30, 2020, stockholders approved a proposal to increase the number of shares under the plan by 2,650,000 shares, for a total of 18,035,000 shares. As of October 2, 2020, there were 3,376,152 shares available for grant under the Plan.

The fair value of each option award is estimated on the date of grant using a Black-Scholes option valuation model applying the weighted-average assumptions noted in the following table. Expected volatilities are based on historical volatility of the Company’s stock. The expected term of options granted is derived from the historical exercises and post-vesting cancellations and represents the period of time that options granted are expected to be outstanding. The Company has calculated an 6% estimated forfeiture rate based on historical forfeiture experience. The risk-free rate is based on the U.S. Treasury yield curve corresponding to the expected term at the time of the grant.

	Three Months Ended						Nine Months Ended
	October 2, 2020			September 27, 2019			October 2, 2020			September 27, 2019
Expected dividend yield		0	%		0	%		0	%		0	%
Expected volatility		53	%		53	%		53	%		53	%
Risk-free interest rate		0.31	%		1.55	%		0.53	%		2.40	%
Expected term (in years)		5.72			5.67			5.72			5.67

A summary of stock option activity under the Plan for the nine months ended October 2, 2020 is presented below:

	Stock Options (in 000’s)			Minimum Exercise Price		Maximum Exercise Price
Outstanding at January 3, 2020		4,326
Granted		616
Exercised		(1,146	)
Forfeited or expired		(14	)
Outstanding at October 2, 2020		3,782		$	5.05	$	51.42
Exercisable at October 2, 2020		2,594

A summary of restricted stock, RSUs and PSUs activity under the Plan for the nine months ended October 2, 2020 is presented below:

	Restricted Stock (in 000’s)			Restricted Stock Units (in 000’s)			Performance Stock Units (in 000’s)
Unvested at January 3, 2020		11			104			—
Granted		11			127			15
Vested		(11	)		(93	)		—
Forfeited or expired		—			(1	)		—
Unvested at October 2, 2020		11			137			15